HP Fundamental/Harmonic Mixer 5086-7285 (22 GHz): digital bias control

In an effort to build a 2-18 GHz down converter, a HP mixer 5086-7285 needs to be controlled. This is one of a group of 22 GHz mixers, all used in earlier HP spectrum analyzers. These mixers are very linear, and useful both at fundamental and harmonic frequencies.

That’s the little magic thing, and the frequency list-harmonics:
5086-7285 mixer
5086-7285 mixer harmonics

All in all, at a first glance, pretty easy to use – it only needs +10 and -10 V power supply and bias for the diode.

Well, bias, after looking through the schematics, this is the assembly taking care of it: a board full of resistors and amplifiers, with no less than 22 (!) adjustment pots.
08565-60023 bias assembly

The interesting part are the bias drivers itself –
hp bias circuit for harmonic mixer
– the linearization, etc., this can all be done easily by using digital memory and a DAC nowadays, but the drivers, we still need them.

The bands B3 and B5, the even harmonics, the things are clear and as expected – a voltage source, and a resistor. Easy enough. But, what did HP do for the odd harmonics?? – the are a few extra resistors around the opamps, and these resistors make it a tricky thing. Too tricky to make it easy to understand. Some kind of negative resistance circuit/kind of a voltage to current converter, which depends a bit on the load resistance.

So, what do you do to understand such things better – build a little test circuit, here we go:
mixer bias test circuit
-it is essentially the same circuit, as for the B1/B4/B2 bands, U6B of the HP circuit- just left out the switching transistor.

It works pretty well, and as a U to I converter, see here:
bias driver test 200 mv-div ramp  1 mA-div current
– ramp voltage is the drive signal, 800 mV p-p, 200 mV per div (center line is zero). During the negative signal period, the output is active – current signal is 1 mA per div (center line is zero).

Having the basic functionality of the ciruit confirmed – some calculations with LTSpice, one of the best general purpose analog simulators around, Thank You, Linear Technology!

Here the files, in case you want to investigate it yourself:
hp mixer bias

This is a typical result, mixer bias current, vs. input voltage of the circuit, at resistance (of the mixer), of 950 (steepest)-1050-1150-1250 ohms.
r6-92 1-9 bias rscan vs Vi
So, this cirucit really is a U to I converter, with the slope depending on the load resistance.
Also note the model circuit of the mixer internal resistor and diodes. The two diodes and the 970 Ohm resistor are the result of bias current vs. bias voltage measurement. Bias voltage is in the range of -1 to -7 volts, about 0 to 8 mA.

With these findings, next step will be to build a driver circuit that can work fully digitally controlled, with no adjustment pot at all (series resistors will be manually selected).

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